Switch – replace 1 switch of a 3-switch box with a Motion Detector switch

Most motion detectors have a built-in override:

• Normally the switch is kept on all the time. The motion detector monitors ambient light (daylight) and then activates when it's dark enough, and motion is detected.
• If you turn the switch off and on within a second or so, the light will stay on, and this overrides motion detection.
• To go back to normal operation, turn the switch off and wait ~10 seconds, then turn back on.
• I believe most will also go back to normal operations after some hours, or after the sun comes up again

It is possible that you'll damage the electronics by overriding the detector.

The safe way to do this would be to use a relay.

You can probably replace the switches with those sensors, but you're going to have to do some creative wiring to work around the fact that they're not 3-way switches, but the switches you're replacing with them are. You'll likely end up with the light being on whenever either of the switches is on. You also might run into trouble if they ran the 3-way switches as a switch loop (no neutral in box), because "smart switches" like these require a neutral so they can use power for themselves to decide when to switch on and off.

Here's what the difference between these wiring schemes looks like if you draw it out:

Also note that those occupancy sensors still have a hard-on switch, so your tenants can still just leave the lights on. You might want to look instead at vacancy sensors. (An example vacancy sensor is Cooper Wiring Devices K01-400RW-L, but note that it does not handle CFLs, just incandescents.) These, you tap to turn them on, and they turn themselves off automatically when everyone's gone. That sounds more like what you want.

The how for switch #1 depends again on how they wired your #1 switches and the light they control together. If you diagram out the connections, it becomes straightforward. Until you have enough info to diagram it out, you don't have enough info to rewire it, either.

As an aside, all the exterior stairwell lights I've seen in apartment complexes were on timers. They'd come on around dark and flip off in the morning. I knew they were timers rather than photosensors because a couple times a year they'd end up slightly out of sync with actual daylight hours, but if these are in fact exterior lights, putting them on a photosensor and disabling the switches entirely might be an option.